Sparc64 is unique on linux in *not* passing ucontext_t as
the third argument to a SA_SIGINFO handler.  It passes the
old struct sigcontext instead.

Set both pc and npc in host_signal_set_pc.

Fixes: 8b5bd461 ("linux-user/host/sparc: Populate host_signal.h")
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>